Fuel cell anode

ABSTRACT

The invention refers to a fuel cell electrode for oxidation of preferably methanol in a fuel cell with acid electrolyte. The electrocatalytically active material comprises known noble metal catalysts such as mixtures of platinum and ruthenium which have been fortified with lead or lead compounds added in this connection in a large quantity so that the addition of lead or lead compounds is above a concentration around 5 mg Pb/cm 2 . A suitable amount is 5-10 mg Pb/cm 2 . Fuel cell electrodes of this kind can also be used for other fuels for instance hydrogen containing carbon monixide in acid electrolytes up to around 250° C.

A fuel cell which directly converts the chemical energy in a liquid fuelto electricity in the cold combustion process taking place in a fuelcell has for a long time been considered the final goal for the fuelcell researchers. Several very large development projects have beenconcerned with this task. Methanol has been the fuel of choice but otherfuels like diesel oil or organic compounds with high energy density likeethylene diglycol have also been tested. These efforts have not beenentirely unsuccessful. By means of heavy load of noble metal catalystscorresponding to about 300 mg platina/W it has been possible todemonstrate a performance corresponding to the level 20-30 mW/cm²electrode area. This performance level is satisfactory in specialapplications like cordless chargers. Such generators, however, becomevery expensive because of the noble metal content.

This kind of fuel cells uses PTFE bonded carbon electrodes with theactive catalytic material disposed on the carbon support. The airelectrode is a conventional gas diffusion electrode with 0.5-1 mgplatinum/cm² The methanol electrode is using PTFE only as a binder notas a hydrophobic agent. The catalytic material is frequently a 50/50mixture of platinum and ruthenium. It has also been found that smalladditions of lead, tin and bismuth as an alloy component or as so calledadatoms improves the catalytic action somewhat. The electrolyte istypically 4-M sulphuric acid and the operating temperature 50°-60° C.Methanol is added to the electrolyte from a tank by means of a pump.Carbon dioxide which is formed in the cold corbustion process evaporatesfrom the acid electrolyte.

The air electrode is frequently sensitive to the presence of methanoland may be protected by means of an ion exchange membrane. Fuel cells ofthis kind are described in U.S. Pat. No. 4,262,063. The state of art isdescribed in B.D. McNicol and D.A.J. Rand: "Power Sources for ElectricVehicles", Elsevier, Amsterdam 1984, p. 807-836. The reason for the highquantities of noble metal required in state of art methanol electrodesis considered to be poisoning by intermediates or by-products of theelectrode reaction. A methanol electrode shows initially a very highactivity. This high initial activity is lost already within 50milliseconds. Several studies have been carried out to clarify thepoisoning effect but the results are not entirely conclusive. Manyresearchers, however, maintain that the poisoning can be attributed toabsorption of carbon monoxide which also is in harmony with experiencesfrom other types of fuel cells with acid electrolyte and noble metalcatalyst.

The present invention is the result of an unconditional evaluation offairly large quantities of different chemical compounds which have beenchosen by intuition with no regard to any views regarding reactionmechanism. This search has been successful and resulted in a methanolelectrode with very much better performance and therefore very muchlower material costs than the methanol electrodes which have beenbriefly described above. In this way the most important obstacle for theuse of the direct methanol air cell in practice has been eliminated. Ithas also been shown that the new electrode is useful also with otherfuel cells and fuel cell types which utilize acid electrolytes in atemperature region up to about 250° C.

The invention is characterized in that it refers to a fuel cell anodewith noble metal catalyst for fuel cells with acid electrolytecharacterized in that the anode contains a separate addition of lead orlead compounds in a concentration above about 5 mg Pb/cm².

The voltage current curve for a methanol electrode with lead addition isimproved with more than 200 mV compared to a corresponding electrodewithout lead addition. No explanation to this can be offered at thepresent stage. As mentioned above lead in the form of adatom or alloyingcomponent has a certain positive effect on the performance of themethanol electrode but this effect is small compared to the effect ofthe lead additive according to the invention. Of course the leadadditive according to the invention can be combined with the methanolelectrodes with a noble metal alloy containing platinum-ruthenium withtin, lead or bismuth additive according to the present state of art.

Alloying with lead or coating with lead adatoms modifies the propertiesof the noble metal surface in a favourable way for the oxidation of themethanol. Such a mechanism could not be the explanation to the dramaticimprovement which is obtained with the separate large lead additions tothe electrode structure according to the present invention. There mustbe some kind of symbios between the noble metal catalyst and the largeaddition of lead since lead alone has no catalytic action.

There is a possibility that the lead addition takes care of orcontributes to the current transport. It is also possible that the noblemetal catalyst supports, in one way or another way, a reduction of Pb²⁺to Pb° during simultaneous oxidation of methanol and/or intermediateproducts occurring at the direct electrochemical oxidation of methanol.To clarify the mechanism for the technical action of the invention is avery interesting research topic.

The invention will now be described in detail by means of some examples.Efficient noble metal catalyst for methanol electrodes can be producedby means of so called microemulsion technique. A typical such electrodeaccording to the present state of art is manufactured in the followingway: 516 g normal-hexan and 60 g of a surface active agent, Berol 50,were mixed well in a Braun-mixer. Thereafter two portions of distilledwater of 12 ml were added during agitation at a high velocity. 12,5 mldinitrodiamine platinum (II}-salt solution and 9.5 rutheniumnitrosylnitrate salt solution were added with vigorous agitation. The pHhas adjusted to pH 5-6 with Na-oktylate. The noble metal was out-reducedby means of hydrazin hydroxide. 10 g Shawnigan carbon was added after 15minutes, the mixture was stirred during 2 minutes. The carbon wasprecipitated after 10 minutes with 10 ml tetrahydrofuran. After wash anddrying 5 % polytetrafluorethylene was added as a dispersion. Afterdrying at 40° C. and sintering at 320° C. during 30 minutes 1.5 g PbOwas mixed into the mass which was rolled on a carbon paper.

The lead addition can be incorporated in such an electrode in differentways. One may use the technique which is developed for the manufactureof commercial lead acid batteries. After all one may visualize the fuelcell anode according to the invention as a physical mixture of aconventional fuel cell anode and the electrode material for aconventional negative lead electrode. A simple method is just to mix inlead oxide powder of battery quality as in this example. However, onemay also use other lead compounds like PbS04, PbS, metallic lead powderetc. After a short time in the cell environment these lead compoundsshow the same activity as the lead oxide in the present example.

FIG. 1 shows a voltage current curve for different methanol electrodesagainst a reference electrode in 4-M sulphuric acid with an addition of1-M methanol. The electrode with 0.7 mg Pb/cm² gives a good initialactivity but is loosing the activity after 1 week. 1 mg Pb/cm² gives alasting action as higher quantities do.

The key to the technical effect of the invention is thus thecomparatively large quantity of added lead compound. Here a level around1 mg Pb/cm² has been indicated with reference to FIG. 1 as the regionwhere the technical effect of the invention is fully developed. Atechnical effect comparable to the effect of adatoms is apparently alsoobserved somewhat below this level. This effect, however, is lost aftercomparatively short time and is therefore of no value for practicalapplication. The reduction can to some extent be caused by the fact thatPbSO₄ has a certain solubility in the electrolyte at this temperature.This solubility does not, however, influence effects of adatoms or leadpresent in the alloy form.

Lead is a cheap material and it is therefore possible to work in thepractice with comparatively large quantities at the level of 5-10 mgPb/cm² or above. For comparison it can be mentioned that the leadquantities used as adatoms or alloy components can be calculated toconcentrations on the level 0.1-0.3 mg Pb/cm².

Most of these experiments have been carried out with fuel cell anodesfor methanol as described above. It has, however, been found that asimilar technical effect is also obtained with other systems forinstance fuel cell anodes for hydrogen in cells with phosphoric acid aselectrolyte or fuel cell anodes for reformed natural gas with carbonmonoxide in acid electrolyte.

I claim:
 1. Fuel cell anode with a catalyst of a mixture of platinum andruthenium for fuel cells with acid electrolyte characterized in that theanode contains a separate additive of lead or lead compounds in aconcentration above about 5 mg Pb/cm² in a physical mixture with theother electrode components.
 2. Fuel cell anode according to claim 1characterized in that the additive of lead or lead compounds is in theregion of about 5-10 mg Pb/cm².